1. Technical Field
The present invention relates to the conversion of waste rubber material, such as used tires or other scrap materials, into a product suitable as a component in the production of other rubber materials and in particular to the apparatus and methods for the devulcanization of the rubber to allow its subsequent use as a virgin polymer substitute.
2. Description of Related Art
The disposal of used rubber continues to pose an environmental challenge and the development of methods for its conversion to a useful material has long been the focus of much investigation. One large source of scrap rubber is found in used tires as an example, which are not biodegradable and therefore present a lasting threat to the environment, especially when stockpiled in land fills. Those concerned with the environmental impact of overstocked landfills have sought ways to reduce the growing stockpile of discarded tires or other scrap rubber material. Consequently, technology for the treatment of used rubber materials such as tires, as an alternative to dumping has been the focus of much recent investment and scientific investigation. In addition to sources of scrap rubber such as tires, large volumes of additional scrap materials are produced in manufacturing processes, or in the scrapping of other post consumer materials from a variety of different products including automobiles and a wide variety of other products and processes. These rubber materials can include all varieties of rubber including EPDM, SBR, natural latex, chlorinated Butyl rubber, chloroprene or the like. Many of these materials may be found exclusively in a particular product, or may be produced as scrap material in an original manufacturing process, or the products may be made from a combination of different rubber materials. Tires as an example may include four or more different rubber materials which make up different parts of the tire.
For use, rubber is vulcanized to obtain increased strength and wear resistance which also then contributes to the indestructibility and non-biodegradability of the material, produced as scrap or after its useful life. For tires as an example, the body of a tire typically comprises rubber tread and rubber coated sidewalls. In addition, tires typically include reinforced belts of steel wire and/or reinforced fabric or fiber. The vulcanization of the rubber materials, as well as the presence of steel belting and fiber, contributes to the difficulty of recycling used tires. Other post-consumer rubber materials may also contain materials other than the rubber itself. Despite these difficulties, the critic need exists for recycling of vulcanized rubber materials, including tires, to allow reuse of the materials in place of virgin materials.
Attempts have been made to reduce the scrap rubber materials to particles for use as a filler in a subsequent manufacturing process or as an ingredient in another compound. In general, as the rubber materials are vulcanized, merely reducing the rubber materials to smaller size particles may not be effective, as the rubber materials are still vulcanized or cured, rendering them unusable for many desirable uses as a filler or in a manufacturing process. Again taking used tires as an example, attempts have been made to use ground up tires as an additive or filler for use in asphalt compositions or other road paving materials. Attempts have also been made to utilize low proportions of scrap rubber along with virgin rubber as a filler to manufacture new tires, although the addition of vulcanized filler materials tends to weaken and destroy the physical characteristics of the final products, such as tires or other particles. In attempts to recycle rubber material such as used tires, a number of physical processes have been developed to reduce the size of tires. Shredders and similar devices which reduce the volume of tires are known. Other physical processing methods include cryogenic crushing processes, washing, and separation of the sidewalls of the tire from the tread. Wholly mechanical methods are also used to remove the rubber tread from the rest of the tire, along with methods to remove the fiber and steel in the tire are known. Cryogenic crushing involves the use of a cryogen such as liquid nitrogen to lower the temperature of the tire and to harden the rubber so that it fractures into smaller pieces when mechanically ground. Although reducing vulcanized rubber to smaller particles by shredders or cryogenic crushing processes can result in a material which can be used for very limited purposes, it would be desirable to further treat the rubber material so as to devulcanize it, such that the devulcanized material could be used in place of virgin materials to a much greater extent. Devulcanization converts rubber from its thermoset, elastic state into a more plastic state which thereafter allows revulcanization, by severing the sulphur bonds in the molecular structure of the devulcanized rubber. The devulcanized rubber may then be used in place of virgin materials in original manufacturing processes.
There are known devulcanization methods used to convert vulcanized materials into reusable materials by chemical means. Typically after an initial physical grinding or particle reducing step the cured rubber is subjected to a process known as chemical digestion, which completely dissolves the fabric which may be in the material and the cured rubber is fragmented into molecules which can be further separated. Chemical processes such as hydrogenation, pyrolysis and other chemical methods used to fragment rubber polymer into smaller molecules are typically costly. The chemical methods are limited to the recovery of small molecules, metal, and other byproducts of the treatment process. Typically, it is not possible to reuse the products of chemical treatment directly as a rubber substitute.
Processes for the conversion of cured scrap rubber into a useable raw material are also known. Typically, the product of such processes is used as a filler or other substitute. A common problem associated with polymer substitutes or fillers is the loss in physical properties, such as tensile strength, accompanying the use of significant percentages of filler material. The particle size of the resulting product relates in part to the loss in physical properties of the filler, so that the smaller the particle size of the filler the more closely the filler retains the physical properties of the pure raw material. A significant decrease in the loss of physical properties in going from the pure raw material to the filler results in a lower percentage of filler which may be substituted for the virgin raw material. The use of larger percentages of filler means lower cost and a higher degree of recycling of the cured rubber scrap.
The development of methods for producing recycled rubber or polymers has focused on producing materials which can be used with virgin materials without a substantial loss of physical properties. A number of these methods involve the use of a chemical solvent to initially swell the scrap rubber which is then mechanically comminuted or otherwise reduced in particle size. However, the use of solvents and other chemical means for breaking down polymer materials is accompanied by the production of by-products which can alter the physical properties of the end product which may require a purification step for their removal. One known process for partially devulcanizing cured rubber materials has been developed by STI-K Polymers, Inc. which utilizes a chemical devulcanizing agent which causes surface devulcanization of the materials. The degree of devulcanization achieved using this process is less than would be desired, and other problems exist with the use of chemical processing agents.
Based upon the foregoing, there remains a need for an improved process for converting cured rubber and other polymeric materials into materials for use as a filler or substitute for virgin materials which does not involve the use of solvents or other chemicals and which can supply an end product which substantially maintains the physical properties of the virgin material.
The present invention therefore relates to processes and apparatus for the production of a virgin polymer substitute from cured scrap rubber, such as automobile tires, which does not involve the use of solvents or other chemicals and which substantially maintains the physical properties of the virgin material in the substitute. The process and apparatus comprises an initial analysis of the scrap material to be treated and determination of the process parameters for that particular material. Actual treatment of the material comprises grinding of the scrap rubber and preconditioning the ground material by subjecting it to warm dry air prior to subsequent treatment in a high temperature and pressure environment. The high temperature and pressure environment is created by means of a pressure vessel into which the preconditioned material is introduced, thereafter exposing the material to a variable high pressure and temperature environment which is controlled to optimize devulcanization of the material. Continuous agitation of the material during treatment serves to insure uniform heating and exposure of materials, thereby producing a substitute which can be used without substantial diminution in physical properties such as tensile and tear strength.
Accordingly it is a principal object of the present invention to provide a process and apparatus for the conversion of cured scrap rubber materials, such as vehicle tires, into reusable materials, which can be introduced into a variety of products and processes without substantial loss of physical properties.
The above and other objects, features and advantages of the present invention will become apparent from the following description read in conjunction with the accompanying drawings, in which like reference numerals designate the same elements.